1. Field of the Invention
The present invention relates to a bandwidth extension apparatus and a method therefor, particularly for expanding the bandwidth of a sound signal having its frequency band limited by generating and adding a frequency component higher than the upper limit of the frequency band.
2. Description of the Background Art
For example, telephone communication systems conveying speech signals have the frequency band thereof limited to the range from 0.3 kHz to 3.4 kHz, which is so much narrower than the frequency range of genuine human voices. Therefore, the quality of speech signals transmitted over telephone systems is somewhat deteriorated to the level of muffled voice.
In order to solve the above-described problem, some solutions have been proposed to improve the quality of received speech signals by extending the frequency band of the received speech signals to its higher frequency region. One of the solutions is disclosed, for example, by Japanese patent laid-open publication No. 2000-134162 to Omori, et al.
In the method disclosed in Omori, et al., a received speech signal, i.e. lowband signal, is used to generate a signal component higher in frequency than the lowband signal, and the latter component, i.e. highband signal, is added to the lowband signal to thereby produce a speech signal having its frequency band wider than the bandwidth of the received signal. The method of Omori, et al., is characterized in that the additive ratio of highband signal to lowband signal is adjustable by manual control from the outside.
However, the method described in Omori, et al., involves a problem that the manual adjustment of the additive ratio requires much more time and higher skill, and is therefore rather difficult for ordinary users.
Among various bandwidth extension solutions, there is one in which the user or service engineer can adjust and control the characteristics for use in generating a highband signal. It is, however, also difficult for them to do the adjustment and control effectually.